Wavelength dependence of solar irradiance enhancement for 61 X-class flares that occurred between 1989 and 2012 is investigated.

FISM is coupled to TIE-GCM to study influence of individual solar waveband to upper atmosphere for flare on October 28th, 2003.

For the 61 X-class flares, the 0–14 nm irradiance enhanced 680%, while the 25–105 nm irradiance increased 120% on average.

The largest neutral temperature and density perturbations at 400 km are due to irradiance enhancement in 25–105 nm.

High-altitude thermospheric response to flare irradiation in 0–175 nm is almost a linear combination of the responses to individual wavebands.